Integrated polarization beam splitter with quarter-wave plate for polarimeter and PMD compensation applications

ABSTRACT

An optical device for changing polarization comprises a waveguide having a waveguide end facet coupled to a quarter-wave plate-reflector combination to rotate the polarization of incident light to the waveguide by 90 degrees. In one embodiment, a polarization beam splitter/rotator combination (PBSR) uses a quarter-wave plate in reflection at the end facet of the waveguide. The polarization beam splitter/rotator combination and variations of that structure are applied in various useful topologies as polarization mode dispersion (PMD) compensators and polarimeters.

FIELD OF THE INVENTION

[0001] This invention relates a polarization beam splitter andquarter-wave plate combination that is particularly useful forpolarization measurement and PMD compensation.

BACKGROUND OF THE INVENTION

[0002] High-speed optical fiber communication systems operate byencoding information (data) onto lightwaves that typically propagatealong optical fiber paths. Most systems, especially those used formedium to long distance transmission, employ single mode fiber. Asimplied by the name, single mode fibers optimally propagate one mode ofa lightwave. The single mode of light typically comprises manycommunications channels. The many communications channels are combined,or multiplexed into the one transmitted mode, as by wavelength divisionmultiplexing (WDM) or dense wavelength division multiplexing (DWDM).

[0003] While there is only one mode transmitted, that single modeactually comprises two perpendicular (orthogonal) polarizationcomponents. These two components propagate at different speeds along afiber transmission path, producing undesirable distortion of the opticalsignals referred to as polarization mode dispersion (PMD). PMD can becorrected in optical transmission systems using measurements of PMD tocontrol active corrective optics.

[0004] Polarimeters measure the polarization of light, and PMDcompensators correct dispersion. Polarimeters generate signalsrepresenting a measured state of polarization that can then be used forpolarization correction. PMD compensators accomplish PMD correctionbased on the measured signals. These devices can be compactly fabricatedas integrated structures on a substrate.

[0005] An important component in polarization measurement andpolarization compensation is a component to modify the polarization ofthe light. This component is typically provided as a polyimide halfwaveplate inserted into a grove formed in a silica-based single modewaveguide. (See for example, “Polarization Mode Converter with PolyimideHalf Waveplate in Silica-Based Planar Lightwave Circuits”, Y. Inoue, etal., IEEE Photonics Technology Letters, vol. 6, no. 5, pp. 626-628,1994.) The waveplate is provided by dicing or etching a trench across awaveguide and then epoxying the half-wave plate in the gap. Thedifficulty with this approach is that the trench often cuts across otherwaveguides, adding unnecessary loss and backreflection. Also, typicalpolyimide half-wave plates have 40 micron widths. While, such widths areacceptable for low index waveguides, with high index waveguides theseveral decibel (dB) diffraction loss is not tolerable.

[0006] Accordingly there is a need for an improved component to modifythe polarization of light, especially for use in polarizationmeasurements and PMD compensators.

SUMMARY OF THE INVENTION

[0007] An optical device for changing polarization comprises a waveguidehaving a waveguide end facet coupled to a quarter-wave plate-reflectorcombination to rotate the polarization of incident light to thewaveguide by 90 degrees. In one embodiment, a polarization beamsplitter/rotator combination (PBSR) uses a quarter-wave plate inreflection at the end facet of the waveguide. The polarization beamsplitter/rotator combination and variations of that structure areapplied in various useful topologies as polarization mode dispersion(PMD) compensators and polarimeters.

DRAWINGS

[0008] The advantages, nature and various additional features of theinvention will appear more fully upon consideration of the illustrativeembodiments now to be described in detail in connection with theaccompanying drawings. In the drawings:

[0009]FIG. 1 shows a PBS plus quarter-wave reflector rotator configuredas a PBSR;

[0010]FIG. 2 shows a PBSR with an integrated quarter-wave plate andexternal reflector;

[0011]FIG. 3 shows an exemplary implementation of a 1×2 PBSR; 20

[0012]FIG. 4 shows an exemplary compensator using a PBS and quarter-waveplate-reflector combination;

[0013]FIG. 5A shows one embodiment of a compensator using ringresonators and a PBS and quarter-wave plate-reflector combination;

[0014]FIG. 5B shows a second embodiment of a compensator using a ringresonator and a PBS and quarter-wave plate-reflector combination;

[0015]FIG. 6 shows one embodiment of a spectral polarimeter using aquarter-wave plate-reflector combination;

[0016]FIG. 7 shows an efficient polarimeter using a polarization beamsplitter rotator (PBSR);

[0017]FIG. 8 shows another embodinent of an efficient spectral(wavelength dependent) polarimeter; and

[0018]FIG. 9 shows an embodiment of Stokes polarimeter withspectrally-resolved input section.

[0019] It is to be understood that the drawings are for the purpose ofillustrating the concepts of the invention, and except for the graphs,are not to scale. It is also understood that many of the opticalcomponents shown in discrete form can be in integrated form, includingintegrated wave plates, polarizers, and mirrors as known in the art.Also, it is understood that where one phase shifter is shown in only onearm of a tuneable coupler, MZI, or two parallel waveguides going into acoupler, a second phase shifter can be added to the other arm.

DETAILED DESCRIPTION

[0020] The detailed description is divided into two parts. Fist, theinventive PBS/rotator structure is introduced. Part I shows PMDcompensators using the inventive PBS/rotator structure. Part IIdiscusses various polarimeter topologies, also using the inventivestructure.

[0021] Part I: The PBS/Rotator Structure (PBSR

[0022]FIG. 1 shows a new configuration 10 for a polarization beamsplitter (PBS) plus rotator structure, referred to herein as “PBSR”.Light is input to PBSR 10 at port 11 of 2×2 PBS 14. Quarter-wave plate15 is coupled to the end facet of the waveguide 17, the through port of2×2 PBS 14. A reflector 16, such as a mirror or Bragg Grating, iscoupled to Quarter-wave plate 15. Quarter-wave plate 15 is oriented atabout 45 degrees to the birefringent axis of waveguide 17. PBSR 10outputs orthogonal polarizations at ports 12 and 13 as co-polarizedlight. The outputs are co-polarized because the double pass throughquarter-wave plate 15 causes one polarization to be rotated by 90degrees and delayed as compared to the other polarization. It isequivalent to a single pass transmission through a half-wave plate.

[0023] The use of a mirror at the end facet of waveguide 17 avoids theneed for a trench and careful alignment of the waveplate in a trench.Such trenches often cut across other waveguides, adding unnecessary lossand backreflection. Quarter-wave plates 15 with 8 microns thickness areavailable, thus the double-pass distance is less than half that for thehalf-wave plate, substantially reducing diffraction losses, and thewaveplate can easily be attached to the polished end facet of thewaveguide without interfering with other waveguides on the circuit.Waveplate 15 can easily be attached to the polished end facet ofwaveguide 17 without interfering with other waveguides on the circuit,and final alignment of the axis of waveplate 15 relative to thebirefringent axis of waveguide 17 can be more easily accomplished.

[0024] The one-input/two-output (1×2) PBSR is shown in integrated form,in FIG. 1, where the dotted box indicates the planar waveguide portion.It can also be operated in reverse (2×1). The optical path lengths ofthe two output ports are designed to be equal. Quarter-wave polarizationrotator 15 can also be integrated as shown in FIG. 2. In this case, thetolerances on the rotation angle and equivalent waveplate thickness aregreatly relieved by passing the signal back through PBS 14, with highextinction of the undesired polarization, after reflection.

[0025]FIG. 3 is an exemplary implementation of a 1×2 PBSR. Light isinput to the structure at PBS port 11. Orthogonal polarizations areoutput at PBS port 12 and cross-port 13. Light reflected by mirror 16 atthe through port leading to waveguide 17 (with ½λ rotation as a resultof two passes through quarter-wave plate 15) crosses and exits at output12. The orthogonal polarization is output at port 13. Mode transformers(not shown) can be used, for example, in waveguide 17 to reducereflected light loss from the quarter-wave plate 15/reflector 16combination. Tunable phase shift 33 can be used to adjust the delaybetween the two legs. The cross-port of a 1×2 PBS has a much higherextinction ratio than the thru port because the couplers are not exactly50/50 splitters. This extinction ratio asymmetry is advantageously usedin the double-pass configuration to improve the lower extinction port.The optical path lengths of the two output ports are designed to beequal.

[0026] Polarization Diversity Applications (PMD Compensators)

[0027] An integrated PMD compensator and tunable chromatic dispersioncompensator can be implemented comprising PBS 14 and a quarter-waveplate 15 reflector 16 combination, as shown in FIG. 4. The tunableallpass filters, shown as ring resonators 41, compensate dispersion andfirst-order PMD. The allpass filters are advantageously double-passedbecause of the light reflected from mirror 16 serving as a reflector.

[0028] It should noted that if the birefringence is too large, thefilter passbands will not overlap. In this case, a mirror can be usedinstead of the quarter-wave/reflector combination and a circulator isthen needed to separate the input and output signals since they are notautomatically separated, as is the case in FIG. 4.

[0029]FIGS. 4, 5A and 5B show architectures suitable for birefringentnarrowband filters, such as ring resonators 41 implemented in planarwaveguides, with a quarter-wave plate 15 reflector 16. This type offilter allows the narrow bandpass responses from both polarizations tobe combined at output 52. By placing polarizer 55 at the output, the TEor TM energy of the two frequencies represented as pulses 54 can bemixed on a photodetector (not shown). The beat frequency phase providesdispersion information. Thus, the birefringence provides a stable beatfrequency as the narrowband filter is tuned. By varying thebirefringence during fabrication, the beat frequency can be set. Onefabrication technique is to ablate a stress-inducing film on top of theupper cladding until the desired birefringence is achieved.

[0030] Polarization State Generators and Analyzers (Polarimeters):

[0031] Polarimeters can be implemented with four detectors in astationary architecture, or a single detector and a rotating waveplate,using the Stokes space representation. A Jones vector approach has beendemonstrated in planar waveguides using four detectors (“Apparatus andmethod for measurement and adaptive control of polarization modedispersion in optical fiber communications systems”, U.S. patentapplication Ser. No. 10/180,842, by C. K. Madsen, which is incorporatedby reference herein), or more than four detectors (See: T. Saida, et.al., “Integrated Optical Polarisation Analyser on Planar LightwaveCircuit,” Electronics Letters, vol. 35, no. 22, pp. 1948-1949, 1999).For sensing applications, a polarization-dependent interferometer hasbeen integrated and referred to as a polarimeter; however, it does notfunction to evaluate the input state of polarization (SOP) as we areusing the tern polarimeter. A critical measurement for polarimeters usedto provide feedback information to PMDCs is to determine the degree ofpolarization (DOP).

[0032] A spectral polarimeter 600 can be obtained without a PBS as shownin FIG. 6, but it requires 4×4 polarizer 608 at the outputs. Light to bemeasured for polarization is input at port 601. Some of the light inwaveguide 602 is coupled directly into ring resonator 604, other lightreflected from the quarter-wave plate 15 and mirror 16 is reflected backand also coupled into ring 604. Splitters 605 are used to create fourlight paths that are coupled directly, or indirectly to polarizer 608and then to detectors D1 to D4. The topmost path 610 is coupled directlyto polarizer 608 and then to detector D1. The next path 611 is coupledto phase shifter 606. Phase shifter 606 is coupled to tunable coupler607, then coupled to polarizer 608 and output to detector D2. Path 612is also coupled to tuneable coupler 607, then coupled to polarizer 608and output to detector D3. And, finally path 613 is coupled directly topolarizer 608 and output to detector D4. Note that the polarizer isaligned with the vertical or horizontal axis of the waveguides and maybe implemented either externally or integrated. Phase shifters 603, 606are used to tune the narrowband filter response and to change therelative phase between polarizations for more robust detection. Tunablecoupler 607 may also be employed to further increase the measurementrobustness for TE or TM input SOPs, which would not provide aninterference signal otherwise.

[0033] An efficient integrated polarimeter architecture that requiresonly a single detector is shown in FIG. 7. Incident light 701 is splitby PBSR 10. The TE component is present on one arm of the PBSR and theTM component is present on the other PBSR arm. One of the components isrotated 90 degrees by the PBSR, therefore both components areco-polarized. Tunable phase shifter, φ₁ 702, changes the relative phasein one arm. Then, both arms are combined in a tunable coupler, which isimplemented using a symmetric Mach-Zehnder interferometer (MZI) 706 withtunable phase shifter φ₂ 704. The output from one arm of the MZI is thendetected by photodetector 705.

[0034] This integrated device is much simpler than the equivalentfour-detector fiber device. It can easily be integrated with numerousother functions, and it relies on fabrication insensitive elements.Tuning can by heat, and the thermo-optic tuning speed can be greatlyenhanced over typical millisecond responses by using high-index contrastwaveguides. Several combinations of phase shifters and tunable couplerscan be used. The only requirement is that the polarization stateanalyzer provides an invertible analysis matrix so that the incomingStokes vector can be determined.

[0035] The equivalent Mueller matrix representation is a linear phaseretarder of retardance φ₁ 702, followed by a retarder oriented at 45degrees having retardance φ₂ 704. By detecting only a single output, weperform the equivalent operation of a polarizer followed by a detector.The Mueller matrix representation, following the notation of, is givenas follows:$S^{out} = {{P_{0^{''}}R_{- 45^{''}}R_{0^{''}}S^{in}} = {{{{\frac{1}{2}\quad\begin{bmatrix}1 & 1 & 0 & 0 \\1 & 1 & 0 & 0 \\0 & 0 & 0 & 0 \\0 & 0 & 0 & 0\end{bmatrix}}\begin{bmatrix}1 & 0 & 0 & 0 \\0 & {\cos \quad \phi_{2}} & 0 & {\sin \quad \phi_{2}} \\0 & 0 & 1 & 0 \\0 & {{- \sin}\quad \phi_{2}} & 0 & {\cos \quad \phi_{2}}\end{bmatrix}}\begin{bmatrix}1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & {\cos \quad \phi_{1}} & {\sin \quad \phi_{1}} \\0 & 0 & {{- \sin}\quad \phi_{1}} & {\cos \quad \phi_{1}}\end{bmatrix}}\begin{bmatrix}S_{0} \\S_{1} \\S_{2} \\S_{3}\end{bmatrix}}}$

[0036] The output intensity is given by

S ₀ ^(out) =[S ₀ +S ₁ cos φ₂ −S ₂ sin φ₁ sin φ₂ +S ₃ cos φ₁ sin φ₂]/2.

[0037] The outputs are determined by measuring four settings of thephase shifter, for example, φ₂=0, φ₂=π, [φ₂=π/2, φ₁=0], [φ₁=−π/2,φ₁=π/2]. Linear combinations of these outputs allow the incoming Stokesvector to be retrieved. The DOP is then calculated in the standardmanner: DOP={square root}{square root over (S₁ ²+S₂ ²+S₃ ²)}/S₀. Acommon drive can be used for both phase shifters with only a fixedamplitude and phase offset between the two phase shifters. For example,−π/4≦φ₁≦3π/4 and 0≦φ₂≦2π. The required maximum phase shift differs by afactor of two and a π/4 offset is evident. For a linear phase change intime, the output is given by:${S_{0}^{out}(t)} = {\quad{\left\lbrack {S_{0} + {S_{1}\quad {\cos \left( {2\quad \pi \quad t} \right)}} - {S_{2}\quad {\sin \left( {{\pi \quad t} - \frac{\pi}{4}} \right)}{\sin \left( {2\quad \pi \quad t} \right)}} + {S_{3}{\cos \left( {{\pi \quad t} - \frac{\pi}{4}} \right)}{\sin \left( {2\quad \pi \quad t} \right)}}} \right\rbrack/2.}}$

[0038] In practice, the maximum phase change applied via thermo-opticphase shifters is limited because of power constraints. A triangulardrive signal with 2π peak-to-peak phase change could be used to providea full characterization of the input polarization twice per period. Therelative phase shift can be fixed in the optical circuit or appliedelectrically. The device in FIG. 7 can also be operated in reverse as apolarization state generator. A light source (not shown) replaces thephotodetector 705 and various polarization states are generated to theleft of PBSR 10.

[0039] An integrated spectral (or wavelength-dependent) polarimeterarchitecture is shown in FIG. 8. The output of the PBSR 10 with a 90degree rotation in one arm is coupled to one side of a tunable ringresonator 802, the narrowband filter, comprising phase shifter 801. Thenarrowband outputs are copolarized so that the birefringence of thewaveguide does not matter. The outputs of the tunable narrowband filterare then operated on as in FIG. 7.

[0040] The architecture in FIG. 9 realizes a Stokes polarimeter. Light908 is input to PBSR 10. The outputs 909 of PBSR 10 are coupled to ring901 comprising phase shifter 902. Ring 901 is coupled to a quarter-waveplate 15 and reflector 16, and a waveguide coupled to 1×4 tap 903. Thefour outputs of the 1×4 tap 903 are coupled via different waveguides todetectors 920-923. One tap 903 output is coupled via waveguide 910 todetector PD4 920. A second tap 903 output is coupled via waveguide 911to waveplate 904 and a polarizer 906, and polarizer 906 is coupled to adetector PD3 921. Another tap 903 output is coupled via waveguide 912 towaveplate 905 and a polarizer 906, and the polarizer 906 is coupled todetector PD2 922. The fourth tap 903 output is coupled to a polarizer906 and to detector PDI 923. The polarimeter has (or three) waveplates904 and 905 and three linear (or four) polarizers 906 that can easily beintegrated. Two waveplates can be a quarter-905 and a half-wave plate904 as shown. However, it is not necessary to precisely implement thesevalues. Any polarization rotations, produced for example by polarizationconverters, which allow the overall characterization matrix to beinverted, are sufficient. Spectral resolution capability can be includedas shown in the dashed box.

What is claimed:
 1. An optical device for separating and changing thepolarization of incident light having a first polarization component anda second orthogonal polarization component comprising: a polarizationbeam splitter (PBS) having, an input port, a bidirectional through port,a cross port, an output port; and coupled to the through port, aquarter-wave plate and a reflector, whereby light from the through portpasses through the quarter-wave plate and is reflected back through thequarter-wave plate into the through port and out through the output portsuch that the second orthogonal polarization component is co-polarizedwith the first polarization component of the incident light.
 2. Theoptical device of claim 1 wherein the PBS comprises a Mach-Zehnderinterferometer (MZI).
 3. The optical device of claim 2 wherein the MZIincludes a tunable phase shifter.
 4. The optical device of claim 1wherein the PBS structure comprises an integrated planar waveguidestructure.
 5. The optical device of claim 1 wherein the reflector andthe integrated quarter-wave plate comprise a quarter-wave plate having afront surface, a back surface and a reflecting layer on the backsurface.
 6. The optical device of claim 1 wherein the reflectorcomprises a mirror.
 7. The optical device of claim 1 wherein thereflector comprises a Bragg grating.
 8. An optical device for changingthe polarization of light comprising: a waveguide having a waveguide endfacet; a quarter-wave plate having a first side and a second side, thefirst side coupled to the end facet; and a reflector coupled to thesecond side of the quarter-wave plate.
 9. A method to change thepolarization of incident light comprising the steps of: receiving thelight; propagating the light via a waveguide having an end facet;transmitting the light from the waveguide end facet through aquarter-wave plate to a reflector; reflecting the light back through theplate and the waveguide.
 10. A tuneable chromatic and polarization modedispersion compensator for compensating incident light comprising: apolarization beam splitter (PBS) having, an input port for receivingincident light, a through port, a cross port, and an output port; afirst series cascade comprising a plurality of ring resonators coupledto the through port and having a first cascade end; a second seriescascade comprising a plurality of ring resonators coupled to the crossport and having a second cascade end; a quarter-wave plate/reflectorcombination coupled to the first cascade end and the second cascade end,the quarter-wave plate/reflector combination receiving light from therespective cascade ends and reflecting the light back into the ends suchthat the light traverses each cascade twice.
 11. The compensator ofclaim 10 wherein the ring resonators are frequency dependent filters.12. The compensator of claim 10 wherein the reflector is a mirror. 13.The compensator of claim 10 wherein the PBS is an integrated opticaldevice.
 14. The compensator of claim 10 wherein the entire structure isan optical integrated device.
 15. A polarization mode dispersion (PMD)compensator for compensating incident light having a first polarizationcomponent and a second orthogonal polarization component comprising: aMach-Zehnder interferometer (MZI) comprising at least two legs andhaving an input port, a through port, a cross port, and an output port,each leg comprising a ring resonator; a quarter-wave plate/reflectorcombination coupled to the through port and the cross port; and apolarizer coupled to the output port.
 16. A polarization mode dispersion(PMD) compensator for compensating incident light having a firstorthogonal polarization component and a second orthogonal polarizationcomponent comprising: a ring resonator; a first optical coupler coupledto the ring resonator, the first coupler further having at least twoports; a second optical coupler coupled to the ring resonator, thesecond coupler having at least two ports; a quarter-wave plate/reflectorcombination optically coupled to at least one port of the first couplerand at least one port of the second coupler; a polarizer opticallycoupled to at least one port of the second coupler; and a photodetectoroptically coupled to the polarizer.
 17. A method of measuringpolarization mode dispersion, comprising the steps of: receiving lighthaving two orthogonal polarization components; separating the twoorthogonal components; copolarizing the two orthogonal components; andmixing the copolarized components on a photodetector to generate a beatsignal.
 18. A method of compensation comprising the steps of measuringin accordance with claim 17 comprising the method step of varying thebirefringence of the at least one ring resonator during fabrication toset the compensation.
 19. The method of claim 18 further comprising themethod step of ablating the outer cladding of the at least one ringresonator during fabrication to set the birefringence.
 20. A spectralpolarimeter to measure the polarization of light comprising: an inputcoupler having a first port and a second port, the first port forreceiving the light; a quarter-wave/reflector combination coupled to thesecond port; a ring resonator coupled to the input coupler between thefirst port and the second port, the ring resonator having a tunablephase shift; a second coupler having a first port and a second port, thesecond coupler coupled to the ring resonator between the second couplerfirst port and the second coupler second port; a first splitter coupledto the second coupler first port, the first splitter having a firstsplitter first port and a first splitter second port; a second splittercoupled to the second coupler second port, the second splitter having asecond splitter first port and a second splitter second port; a phaseshifter coupled to the first splitter second port; a 2×2 tunable couplercoupled to the phase shifter and the second splitter first port; a 4×4polarizer coupled to the first splitter first port, the two 2×2 tunablecoupler, and the second splitter second port, and one or more detectorscoupled to the polarizer.
 21. A polarimeter to measure the polarizationof light having two orthogonal polarization components comprising: apolarization beam splitter/rotator (PBSR) to receive the light at a PBSRinput and to separate the orthogonal polarization components, the PBSRhaving a first PBSR output and a second PBSR output, each of the PBSRoutputs transmitting one of the separated polarization components,wherein the two orthogonal polarization components are copolarized aspresented at the PBSR outputs; a first tunable phase shifter coupled tothe first PBSR output; a Mach-Zehnder interferometer (MZI) having afirst input port, a second input port, a through port, and a cross port,the first input port coupled to the phase shifter, the second input portcoupled to the second PBSR output, the through port coupled to aphotodetector, the cross port optionally having no connection, the MZIhaving two arms, with one arm having a second tunable phase shifter. 22.The polarimeter of claim 21 wherein the phase shifters are heat tunable.23. The polarimeter of claim 21 wherein the phase shifters are tunableto a series of discrete phase shifts.
 24. The polarimeter of claim 21wherein the phase shifters are tunable to a series of discrete phaseshifts by a common control signal.
 25. The polarimeter of claim 21wherein the phase shifters are continuously tuneable.
 26. Thepolarimeter of claim 25 wherein the phase shifters are continuouslytuneable by a triangle wave control signal.
 27. The polarimeter of claim21 wherein the photodetector readings provide the elements of aninvertible matrix for the determination of a Stokes vector.
 28. Thepolarimeter of claim 21 further comprising between the PBSR and thefirst phase shifter and interferometer: a first two port coupler havinga first port and a second port, the first two port first port and thefirst two port second port coupled to the first PBSR output and thesecond PBSR output; a ring resonator, coupled to the two port couplerbetween the first port and the second port, the ring resonatorcomprising a third phase shifter; and a second two port coupler having afirst port and a second port, coupled the ring resonator between thefirst port and the second port, the second two port coupler first portcoupled to the first phase shifter, and the second two port couplersecond port connected to the second input port of the MZI.
 29. Apolarization state generator comprising: a light source; a Mach-Zehnderinterferometer (MZI) having a first input port, a through port, a crossport, and a second input port, the MZI first input port opticallycoupled to the light source, the second input port optionally having noconnection, and the MZI having two legs, wherein at least one legcomprises a phase shifter; a second phase shifter optically coupled tothe through port of the MZI; and a polarization beam splitter/rotator(PBSR) having a first input and a second input and one output, the firstinput optically coupled to the second phase shifter, the second inputoptically coupled to the cross port of the MZI, wherein polarized lightis presented at the PBSR output.
 30. A method of optical polarimetry tomeasure the polarization of light having two orthogonal polarizationcomponents, comprising the steps of: receiving the light to be measured;separating the light to be measured into its orthogonal polarizationcomponents; copolarizing the orthogonal polarization components;shifting one of the components in phase; interfering one of thecomponents with the other in a Mach-Zhender interferometer (MZI) havinga phase shifter in one arm; detecting the output of the MZI with aphotodetector to obtain a plurality of photodetector readings at aplurality of settings of the phase shifters; and calculating thepolarization from the photodetector readings.
 31. The method of claim 30wherein the step of calculating comprises presenting the photodetectorreadings as the elements of an invertible matrix to determine a Stokesvector.
 32. A Stokes polarimeter for measuring the polarization ofincident light comprising: a polarization beam splitter/rotator (PBSR),the PBSR having an input and a first and second output, the PBSRreceiving the incident light at the input and splitting the light into afirst orthogonal polarization component at the first output, and asecond orthogonal polarization component at the second output; a firstcoupler having a first port and a second port, the first port coupled tothe first PBSR output, and the second port connected to the second PBSRoutput; a ring resonator, the ring coupled to the first coupler betweenthe first port and the second port, the ring comprising a phase shifter;a second coupler having a second coupler first port and a second couplersecond port; a quarter-wave plate reflector combination coupled to thesecond coupler, second port; a 1×4 tap coupled to the second couplerfirst port, the 1×4 tap having a first output, a second output, a thirdoutput, and a fourth output; a first photodetector coupled to the tapfirst output; a half-wave plate coupled to the tap second output; apolarizer coupled to the half-wave plate; a second photodetector coupledto the polarizer; a quarter-wave plate coupled to the tap third output;a second polarizer coupled to the quarter-wave plate; a thirdphotodetector coupled to the second polarizer; a third polarizer coupledto the tap fourth output; and a fourth photodetector coupled to thethird polarizer wherein the four photodetector signals yield four Stokesparameters of the polarization measurement.